In modern communication systems such as wireless and satellite communication systems, frequency-converting devices are used that have to be characterized with regard to their electrical behavior. Typically, this is done by frequency-converting measurements applied on the frequency-converting devices, particularly mixers. Hence, the frequency-converting devices to be characterized are also called devices under test or rather frequency-converting devices under test during the respective frequency-converting measurements.
However, the challenge with frequency-converting measurements is twofold. Two input signals are forwarded or rather inputted into the respective device under test, namely a radio frequency input signal (RF signal) and a local oscillator input signal (LO signal). The output of the device under test is assigned to an intermediate frequency output signal (IF signal). The IF signal outputted has a resulting phase that is a combination of the phase of the RF signal, the phase of the LO signal, and the insertion phase of the respective device under test.
Further, measurement equipment used for the respective measurements, for instance a measurement device such as a network analyzer, may measure the scattering parameters of the frequency-converting device under test. The measurement equipment has two receivers utilized to measure the input signal, particularly the RF signal, and the output signal of the device under test. Accordingly, the phase of the RF signal at the input to the device under test as well as the phase of the IF signal at the output of the device under test are measured by the measurement equipment respectively, particularly by the internal receivers of the measurement device, namely the network analyzer.
These internal receivers each have their own insertion phase being a function of frequency.
Accordingly, the phases of the internal receivers also have a contribution to the measurements in addition to the phase of the RF signal, the phase of the LO signal and the insertion phase of the device under test. In other words, the phases of the internal receivers, the phase of the RF signal, the phase of the LO signal and the insertion phase of the device under test are all involved in the measurements.
The goal of the characterization is to measure the insertion phase of the device under test. Thus, there is a need to identify and remove the impact of the other factors mentioned above. Therefore, the measurement equipment, namely the measurement device, has to be calibrated initially.
This type of calibration is generally known in the state of the art. A reciprocal frequency-converting device may be used during the calibration. This approach is known from US 2015/0177300 A1, for instance, and is hereby incorporated by reference in its entirety.
In this approach, artifacts are identified and removed as long as the local oscillator is fixed in frequency during the calibration and measurement processes. Effectively, the phase of the local oscillator is a single fixed value that does not vary during the calibration or rather the measurement.
However, if the local oscillator is stepped in frequency during the measurement, the above-mentioned approach fails, as a shift in the phase of the local oscillator occurs at the junction of the device under test each time the local oscillator is stepped in frequency. This phase shift of the local oscillator may be caused by the method in which the local oscillator frequency is synthesized in the measurement equipment and/or the electrical length of the transmission line leading from the junction of the device under test to the measurement equipment, namely the local oscillator path.
In addition to the approach mentioned above, another approach is known in the state of the art that requires an external reconverting mixer to complete the measurement setup. The external reconverting mixer solves the problem of the stepping local oscillator, as the phase change in the local oscillator is shared between the device under test and the external reconverting mixer, effectively rationing the phase shift out of the measurement result.
However, the external reconverting mixer has to be customized for each device under test individually. Thus, it is not convenient to work with, as it adds additional complexity to the measurement.
As the second approach requires an external reconverting mixer customized to each device under test, there is still a need for a possibility to characterize a frequency-converting device in an easy and efficient manner while using a local oscillator that is stepped in frequency.